Skip to main content

Full text of "Handbook Of Chemical Engineering - I"

See other formats


PYROMETRY
455
TABLE 14.—CORRECTIONS TO OBSERVED TEMPERATURES FOR PYROMETER USING
RED LIGHT (X = 0.65M, C2 « 14,350)
	Add  corrections below for the following observed temperatures,  degrees										
	Centigrade										
Emissivity											
	700	800	900	1,000	1,100	1,200	1,300	1,400	1,600	1,800	2,000
0.30	55	67	80	95	111	129	148	168	213	264	322
0.40	41	50	60	71	83	96	110	125	158	195	237
0.50	31	37	45	53	62	71	82	93	117	144	175
0.60	22	27	33	39	45	52	59	67	85	104	126
0.70	16	19	23	27	31	36	41	47	59	72	87
0.80	10	12	14	17	19	22	25	29	36	44	54
0.90	5	6	7	8	9	10	12	14	17	21	25
1.00	0	0	0	0	0	0	0	0	0	0	0
Temperature of Glowing Gauze.—An interesting application of the optical pyrometer is for the measurement of the temperature of gauze electrically or otherwise heated. In certain chemical processes, platinum gauze electrically heated is used as a catalyzing agent, and must be maintained at a constant temperature. This is readily done by sighting normally on the surface of the gauze with an optical pyrometer. The observed temperatures may be thus exactly reproduced from day to day. If it is required to convert the observed temperatures into exact true temperatures of the wire forming the gauze the problem is difficult. An approximate solution satisfactory for all industrial work is, however, easily obtained.
We will assume that the mesh of the gauze is sufficiently coarse that multiple reflection between the separate wires is negligible. Let A\ = the fractional part of the total area of the gauze comprised by the wire and A2 = the fractional part of the total area representing the space between the wires. Let E\ = the emissivity of the metal employed and E\ = the effective emissivity of the gauze as a whole, that is, taking into consideration the spaces between the wires which of course are not radiating surfaces. The following equations are readily apparent:
• AiE\ since A\ -f-
= 1
1
S
_ log [A iE\ " 9,588
where # is the true absolute temperature of the wire of the gauze and S is the absolute temperature observed with an optical pyrometer sighted normal to the surface. A platinum gauze commonly employed is number 80-mesh (80 wires to the inch) of 0.003-in. wire. For this gauze Ai = 0.42. The emissivity of bright clean platinum is 0.33. The platinum of this gauze soon becomes somewhat corroded. Possibly an. emissivity of say 0.4 is more nearly the correct value under these conditions. Hence, the effective emissivity of the gauze = AiE\ = (0.42) (0.4) = 0.17. Usually the gauze must be viewed through a glass window. A thin glass window (see below) transmits about 90 per cent of the light falling upon it. Hence, the final effective emissivity, using a glass window, = E\ = (0.17) (0.90) = 0.15. The following table was computed by the formula 1/tf - 1/S = log 0.15/9,588. A similar table for other gauzes may be computed in the manner outlined.